System and method for micro-electromechanical operation of an interferometric modulator

ABSTRACT

An interferometric modulator is formed by a stationary layer and a mirror facing the stationary layer. The mirror is movable between the undriven and driven positions. Landing pads, bumps or spring clips are formed on at least one of the stationary layer and the mirror. The landing pads, bumps or spring clips can prevent the stationary layer and the mirror from contacting each other when the mirror is in the driven position. The spring clips exert force on the mirror toward the undriven position when the mirror is in the driven position and in contact with the spring clips.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/189,690, filed Jul. 26, 2005, which is hereby incorporated byreference in its entirety. U.S. application Ser. No. 11/189,690 is acontinuation-in-part of the following: U.S. application Ser. No.10/909,228, filed Jul. 29, 2004; and U.S. application Ser. No.11/048,662, filed Jan. 27, 2005; both of which are hereby incorporatedby reference in their entireties. U.S. application Ser. No. 11/189,690also claims priority to the following: U.S. Provisional Application No.60/613,466, filed Sep. 27, 2004; U.S. Provisional Application No.60/613,499, filed Sep. 27, 2004; and U.S. Provisional Application No.60/658,867, filed Mar. 4, 2005; all of which are hereby incorporatedherein by reference in their entireties.

BACKGROUND

1. Field of the Invention

This invention relates to microelectromechanical systems for use asinterferometric modulators. More particularly, this invention relates tosystems and methods for improving the micro-electromechanical operationof interferometric modulators.

2. Description of the Related Art

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that selectively absorbs and/or reflects light using theprinciples of optical interference. In certain embodiments, aninterferometric modulator may comprise a pair of conductive plates, oneor both of which may be transparent and/or reflective in whole or partand capable of relative motion upon application of an appropriateelectrical signal. In a particular embodiment, one plate may comprise astationary layer deposited on a substrate and the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

SUMMARY

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, its moreprominent features will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description of Certain Embodiments” one will understand howthe features of this invention provide advantages over other displaydevices.

One aspect of the invention provides an interferometric modulator, whichincludes a first layer, a second layer and a member. The first layerincludes a first reflective planar portion. The second layer includes asecond reflective planar portion located substantially parallel to thefirst reflective planar portion. The second layer is movable between afirst position and a second position. The first position is located at afirst distance from the first layer. The second position is located at asecond distance from the first layer. The second distance is greaterthan the first distance. The member includes a surface that is locatedbetween the first layer and second layer. The member defines one or moregap regions between the first layer and the second layer when the secondlayer is in the first position, wherein the second layer in the one ormore gap regions does not contact either the first layer or the member.

Another aspect of the invention provides a microelectromechanicaldevice, which includes a first surface, a second surface and a thirdsurface. The second surface is located substantially parallel to thefirst surface. The second surface is movable between a first positionand a second position. The first position is located at a first distancefrom the first surface. The second position is located at a seconddistance from the first surface. The second distance is greater than thefirst distance. The third surface is located between the first surfaceand the second surface. The third surface defines one or more gapregions between the first surface and the second surface when the secondsurface is in the first position, wherein the second surface in the oneor more gap regions does not contact either the first surface or thethird surface.

Another aspect of the invention provides a microelectromechanicaldevice, which includes a first layer; a second layer and a plurality ofmembers. The second layer is located substantially parallel to the firstlayer. The second layer is movable between a first position and a secondposition. The first position is a first distance from the first layer.The second position is a second distance from the first layer. Thesecond distance is greater than the first distance. Each of theplurality of members includes a surface located between the first layerand second layer. The plurality of members define one or more gapregions between the first layer and the second layer when the secondlayer is in the first position, wherein the second layer in the one ormore gap regions does not contact either the first layer or theplurality of members.

Still another aspect of the invention provides a microelectromechanicaldevice, which includes a first surface, a second surface and at leastone structure on at least one of the first surface and the secondsurface. The second surface is located substantially parallel to thefirst surface. The second surface is movable relative to the firstsurface between a driven position and an undriven position. The drivenposition is closer to the first surface than is the undriven position.The at least one structure is compressed by the first surface and thesecond surface when the second surface is in the driven position. The atleast one structure provides a force to the second surface when thesecond surface is in the driven position. The force assists movement ofthe second surface from the driven position toward the undrivenposition.

Still another aspect of the invention provides a method of making aninterferometric modulator. The method includes: providing a first layer,forming a second layer and forming a member comprising a surface. Thefirst layer includes a first reflective planar portion. The second layerincludes a second reflective planar portion. The second reflectiveplanar portion is located substantially parallel to the first reflectiveplanar portion. The second layer is movable between a first position anda second position. The first position is at a first distance from thefirst layer. The second position is at a second distance from the firstlayer. The second distance is greater than the first distance. Thesurface of the member is located between the first layer and the secondlayer. The member defines one or more gap regions between the firstlayer and the second layer when the second layer is in the firstposition, wherein the second layer in the one or more gap regions doesnot contact either the first layer or the member.

A further aspect of the invention provides a microelectromechanicaldevice produced by a method. The method includes: providing a firstlayer, providing a second layer and providing a member comprising asurface. The first layer includes a first reflective planar portion. Thesecond layer includes a second reflective planar portion. One of thefirst reflective planar portion and the second reflective planar portionmay be partially reflective. The second reflective planar portion islocated substantially parallel to the first reflective planar portion.The second layer is movable between a first position and a secondposition. The first position is at a first distance from the firstlayer. The second position is at a second distance from the first layer.The second distance is greater than the first distance. The surface ofthe member is located between the first layer and the second layer. Themember defines one or more gap regions between the first layer and thesecond layer when the second layer is in the first position, wherein thesecond layer in the one or more gap regions does not contact either thefirst layer or the member.

A further aspect of the invention provides a method of operating amicroelectromechanical device. Here, the device includes a first layer,a second layer and a member. The second layer of the device is locatedsubstantially parallel to the first layer. The member includes a surfaceintervening between the first layer and second layer. The surface of themember is located between only portions of the first layer and thesecond layer. The method of operating the device includes moving thesecond layer relative to the first layer from an undriven position to adriven position. The driven position is closer to the first layer thanis the undriven position. The method further includes contacting themember with at least one of the first layer and the second layer so asto stop the movement of the second layer at the driven position, themember defining one or more gap regions between the first layer and thesecond layer when the second layer is in the driven position, whereinthe second layer in the one or more gap regions does not contact eitherthe first layer or the member.

A further aspect of the invention provides a microelectromechanicaldevice. The device includes first means for partially reflecting andpartially transmitting incident light and second means for substantiallyreflecting incident light. The device further includes means for movingthe first means relative to the second means between a driven positionand an undriven position. The device further includes means forproviding a separation between the first means and the second means whenthe second means is in the driven position. The driven position iscloser to the first means than is the undriven position. The first meansmay include, for example, a partial mirror surface. The second means mayinclude, for example, a full mirror surface. The means for moving mayinclude, for example, a deformable layer. The means for providingseparation may include, for example, at least one of a bump, a landingpad or a spring clip.

A further aspect of the invention provides a microelectromechanicaldevice. The device includes: first means for partially reflecting andpartially transmitting incident light and second means for substantiallyreflecting incident light. The device further includes means for movingthe first means relative to the second means between a driven positionand an undriven position, and means for applying a force on the secondmeans in a direction toward the undriven position when the second meansis in the driven position. The first means may include, for example, apartial mirror surface. The second means may include, for example, afull mirror surface. The means for moving may include, for example, adeformable layer. The means for applying force may include, for example,a spring clip, or, as another example, a bump or a landing pad thatincludes an elastomeric material.

A still further aspect of the invention provides an interferometricmodulator. The interferometric modulator includes a first layer, asecond layer and at least one bump on the at least one of the firstlayer and the second layer. The first layer includes a first reflectiveplanar portion. The second layer includes a second reflective planarportion that is located substantially parallel to the first reflectiveplanar portion. The second layer is movable between a driven positionand an undriven position. The driven position is closer to the firstlayer than the undriven position. The at least one bump is configured toprevent the first layer and the second layer from contacting each other.

A still further aspect of the invention provides an interferometricmodulator, which includes a first layer, a second layer and at least onelanding pad located between the first layer and the second layer. Thefirst layer includes a first reflective planar portion. The second layerincludes a second reflective planar portion that is locatedsubstantially parallel to the first reflective planar portion. Thesecond layer is movable between a driven position and an undrivenposition. The driven position is closer to the first layer than theundriven position. The at least one landing pad includes a contact areawhere one of the first layer and the second layer contacts while notcontacting the other when the second layer is in the driven position.

A still further aspect of the invention provides an interferometricmodulator. The interferometric modulator includes a first layer, asecond layer and at least one spring member placed between the at leastone of the first layer and the second layer. The first layer includes afirst reflective planar portion. The second layer includes a secondreflective planar portion that is located substantially parallel to thefirst reflective planar portion. One of the first reflective planarportion and the second reflective planar portion may be partiallyreflective. The second layer is movable between a driven position and anundriven position. The driven position is closer to the first layer thanthe undriven position. The at least one spring member is compressible byat least one of the first layer and second layer as the second layermoves toward the driven position. The at least one spring member isconfigured to apply force to the second layer in a direction toward theundriven position when the second layer is in the driven position.

Another embodiment provides a display system comprising aninterferometric modulator, a display, a processor and a memory device.The processor is in electrical communication with the display andconfigured to process image data. The memory device is in electricalcommunication with the processor.

Another embodiment provides a method of making a MEMS device, such as aMEMS device that includes an interferometric modulator. The methodincludes forming a first electrode, depositing a dielectric materialover at least a portion of the first electrode, then removing a portionof the dielectric material from over the first electrode, therebyforming a variable thickness dielectric layer. The method furtherincludes forming a second electrode over at least a portion of thevariable thickness dielectric layer. In an embodiment, a sacrificiallayer is deposited over at least a portion of the dielectric materialthat is over the first electrode. The sacrificial layer and at least aportion of the dielectric material may be removed during a later etchingstep. Another embodiment provides an interferometric modulator made bysuch a method.

Another embodiment provides a method of making an interferometricmodulator. The method includes forming a first electrode and depositinga dielectric layer over at least a portion of the first electrode. Themethod further includes removing a portion of the dielectric layer toform a variable thickness dielectric layer, depositing a sacrificiallayer over the variable thickness dielectric layer, planarizing thesacrificial layer, and forming a second electrode over the sacrificiallayer. Another embodiment provides an interferometric modulator made bysuch a method.

Another embodiment provides a method of making an interferometricmodulator. The method includes forming a first electrode and depositinga dielectric layer over at least a portion of the first electrode. Themethod further includes removing a portion of the dielectric layer toform a variable thickness dielectric layer, depositing a sacrificiallayer over the variable thickness dielectric layer, depositing aplanarization layer over the sacrificial layer, and forming a secondelectrode over the planarization layer. Another embodiment provides aninterferometric modulator made by such a method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 5A and 5B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 2.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIG. 8 is a perspective view of an interferometric modulator array whichuses micro-electromechanical system technology.

FIG. 9A is a schematic cross-sectional view of the interferometricmodulator array of FIG. 7 taken along line 8A-8A of FIG. 7.

FIG. 9B is a schematic cross-sectional view of another embodiment of theinterferometric modulator array utilizing micro-electromechanical systemtechnology.

FIG. 10A is a side cross-sectional view of an embodiment of theinterferometric modulator including landing pads with the modulatorshown in the undriven state.

FIG. 10B is a side cross-sectional view of the embodiment of FIG. 9A inthe driven state.

FIGS. 10C-10I are side cross-sectional views of embodiments of theinterferometric modulator, illustrating various configurations oflanding pads.

FIG. 10J is a top cross-sectional view of an embodiment of theinterferometric modulator taken along line 9J-9J of FIG. 9A andillustrating various shapes of landing pads.

FIG. 11 is a flowchart illustrating a method of manufacturing a MEMSdevice having a variable thickness dielectric layer.

FIG. 12 is a cross-sectional view schematically illustrating analternative embodiment of a MEMS device having a variable thicknessdielectric layer.

FIG. 13 is a cross-sectional view schematically illustrating theformation of a lower electrode 502 in accordance with an embodiment.

FIG. 14 is a cross-sectional view schematically illustrating theformation of a dielectric layer 540 (including a lower portion 550 andan upper portion 560) on the stationary layer 502 and over the substrate500 of FIG. 13.

FIGS. 15 and 16 are cross-sectional views schematically illustrating theformation of a variable thickness dielectric layer 570 (including“stops” 565) on the stationary layer 502 of FIG. 13 by removing parts ofthe upper portion 560 of dielectric layer 540 of FIG. 14.

FIG. 17 is cross-sectional views schematically illustrating theformation of a sacrificial layer 710, support structures 720, and anupper electrode 730 of an interferometric modulator.

FIG. 18 is a cross-sectional view schematically illustrating the removalof the sacrificial layer 710 and the removal of parts of the lowerportion 550 of the dielectric layer 570 of FIG. 17.

FIG. 19 shows cross-sectional views schematically illustrating aninterferometric modulator 1800 comprising a stationary layer 502, adeformable layer 506, and a variable thickness dielectric layer 920 thatsubstantially prevents contact between the first electrode 502 and thesecond electrode 506.

FIG. 20 shows cross-sectional views schematically illustrating theformation of a sacrificial layer 710, support structures 720, and anupper electrode 731 of an interferometric modulator.

FIG. 21 is a cross-sectional view schematically illustrating aninterferometric modulator.

FIG. 22A is a side cross-sectional view of an embodiment of theinterferometric modulator with bumps showing the modulator in theundriven state.

FIG. 22B is a side cross-sectional view of the embodiment of FIG. 22A inthe driven state.

FIGS. 22C-22E are side cross-sectional views of embodiments of theinterferometric modulator illustrating various configurations of bumps.

FIG. 23A is a side cross-sectional view of an embodiment of theinterferometric modulator with spring clips showing the modulator in theundriven state.

FIG. 23B is a side cross-sectional view of the embodiment of FIG. 23A inthe driven state.

FIGS. 23C-23F are side cross-sectional views of embodiments of theinterferometric modulator illustrating various configurations of springclips.

FIG. 24A is a side cross-sectional view of one embodiment of a threestate interferometric modulator in the undriven state.

FIG. 24B is a side cross-sectional view of the three stateinterferometric modulator of FIG. 24A in the driven state.

FIG. 24C is a side cross-sectional view of the three stateinterferometric modulator of FIG. 24A in the reverse driven state.

FIG. 24D is a side cross-sectional view of another embodiment of theinterferometric modulator in the undriven state.

FIG. 24E is a side cross-sectional view of another embodiment of theinterferometric modulator in the undriven state.

FIG. 25A is a side cross-sectional view of an alternative embodiment ofan interferometric modulator shown in the undriven state.

FIG. 25B is a top plan view of the interferometric modulator of FIG.25A, shown in the undriven state.

FIG. 25C is a side view of the interferometric modulator of FIG. 25A,shown in the driven state.

FIG. 25D is a top plain view of the interferometric modulator of FIG.20C, shown in the driven state.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theembodiments may be implemented in any device that is configured todisplay an image, whether in motion (e.g., video) or stationary (e.g.,still image), and whether textual or pictorial. More particularly, it iscontemplated that the embodiments may be implemented in or associatedwith a variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

Driving an interferometric modulator may result in contact between adeformable layer and a stationary layer. Such contact may be undesirableand may result in damage to the device, potentially resulting inperformance degradation. Various embodiments provides structures (suchas landing pads, bumps and spring clips) and methods for reducing suchdamage.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise of several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. In some embodiments, the layers are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) deposited on topof posts 18 and an intervening sacrificial material deposited betweenthe posts 18. When the sacrificial material is etched away, the movablereflective layers 14 a, 14 b are separated from the optical stacks 16 a,16 b by a defined gap 19. A highly conductive and reflective materialsuch as aluminum may be used for the reflective layers 14, and thesestrips may form column electrodes in a display device.

With no applied voltage, the cavity 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential difference isapplied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by pixel 12 b on the right in FIG. 1. The behavior is thesame regardless of the polarity of the applied potential difference. Inthis way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5 illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a display array or panel 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the relaxed state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. There is thus a rangeof voltage, about 3 to 7 V in the example illustrated in FIG. 3, wherethere exists a window of applied voltage within which the device isstable in either the relaxed or actuated state. This is referred toherein as the “hysteresis window” or “stability window.” For a displayarray having the hysteresis characteristics of FIG. 3, the row/columnactuation protocol can be designed such that during row strobing, pixelsin the strobed row that are to be actuated are exposed to a voltagedifference of about 10 volts, and pixels that are to be relaxed areexposed to a voltage difference of close to zero volts. After thestrobe, the pixels are exposed to a steady state voltage difference ofabout 5 volts such that they remain in whatever state the row strobe putthem in. After being written, each pixel sees a potential differencewithin the “stability window” of 3-7 volts in this example. This featuremakes the pixel design illustrated in FIG. 1 stable under the sameapplied voltage conditions in either an actuated or relaxed pre-existingstate. Since each pixel of the interferometric modulator, whether in theactuated or relaxed state, is essentially a capacitor formed by thefixed and moving reflective layers, this stable state can be held at avoltage within the hysteresis window with almost no power dissipation.Essentially no current flows into the pixel if the applied potential isfixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4 and 5 illustrate one possible actuation protocol for creating adisplay frame on the 3×3 array of FIG. 2. FIG. 4 illustrates a possibleset of column and row voltage levels that may be used for pixelsexhibiting the hysteresis curves of FIG. 3. In the FIG. 4 embodiment,actuating a pixel involves setting the appropriate column to −V_(bias),and the appropriate row to +ΔV, which may correspond to −5 volts and +5volts respectively Relaxing the pixel is accomplished by setting theappropriate column to +V_(bias), and the appropriate row to the same+ΔV, producing a zero volt potential difference across the pixel. Inthose rows where the row voltage is held at zero volts, the pixels arestable in whatever state they were originally in, regardless of whetherthe column is at +V_(bias), or −V_(bias). As is also illustrated in FIG.4, it will be appreciated that voltages of opposite polarity than thosedescribed above can be used, e.g., actuating a pixel can involve settingthe appropriate column to +V_(bias), and the appropriate row to −ΔV. Inthis embodiment, releasing the pixel is accomplished by setting theappropriate column to −V_(bias), and the appropriate row to the same−ΔV, producing a zero volt potential difference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding, and vacuum forming. In addition, the housing 41 may be madefrom any of a variety of materials, including but not limited toplastic, metal, glass, rubber, and ceramic, or a combination thereof. Inone embodiment the housing 41 includes removable portions (not shown)that may be interchanged with other removable portions of differentcolor, or containing different logos, pictures, or symbols.

The display 30 of exemplary display device 40 may be any of a variety ofdisplays, including a bi-stable display, as described herein. In otherembodiments, the display 30 includes a flat-panel display, such asplasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43 which is coupled to a transceiver 47. Thetransceiver 47 is connected to a processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g. filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28, and to an arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna known to those of skill inthe art for transmitting and receiving signals. In one embodiment, theantenna transmits and receives RF signals according to the IEEE 802.11standard, including IEEE 802.11(a), (b), or (g). In another embodiment,the antenna transmits and receives RF signals according to the BLUETOOTHstandard. In the case of a cellular telephone, the antenna is designedto receive CDMA, GSM, AMPS or other known signals that are used tocommunicate within a wireless cell phone network. The transceiver 47pre-processes the signals received from the antenna 43 so that they maybe received by and further manipulated by the processor 21. Thetransceiver 47 also processes signals received from the processor 21 sothat they may be transmitted from the exemplary display device 40 viathe antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, network interface 27can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a digital video disc (DVD) or a hard-disc drive that contains imagedata, or a software module that generates image data.

Processor 21 generally controls the overall operation of the exemplarydisplay device 40. The processor 21 receives data, such as compressedimage data from the network interface 27 or an image source, andprocesses the data into raw image data or into a format that is readilyprocessed into raw image data. The processor 21 then sends the processeddata to the driver controller 29 or to frame buffer 28 for storage. Rawdata typically refers to the information that identifies the imagecharacteristics at each location within an image. For example, suchimage characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40.Conditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. Conditioning hardware 52 may be discrete componentswithin the exemplary display device 40, or may be incorporated withinthe processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, driver controller 29is a conventional display controller or a bi-stable display controller(e.g., an interferometric modulator controller). In another embodiment,array driver 22 is a conventional driver or a bi-stable display driver(e.g., an interferometric modulator display). In one embodiment, adriver controller 29 is integrated with the array driver 22. Such anembodiment is common in highly integrated systems such as cellularphones, watches, and other small area displays. In yet anotherembodiment, display array 30 is a typical display array or a bi-stabledisplay array (e.g., a display including an array of interferometricmodulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, input device 48 includesa keypad, such as a QWERTY keyboard or a telephone keypad, a button, aswitch, a touch-sensitive screen, a pressure- or heat-sensitivemembrane. In one embodiment, the microphone 46 is an input device forthe exemplary display device 40. When the microphone 46 is used to inputdata to the device, voice commands may be provided by a user forcontrolling operations of the exemplary display device 40.

Power supply 50 can include a variety of energy storage devices as arewell known in the art. For example, in one embodiment, power supply 50is a rechargeable battery, such as a nickel-cadmium battery or a lithiumion battery. In another embodiment, power supply 50 is a renewableenergy source, a capacitor, or a solar cell, including a plastic solarcell, and solar-cell paint. In another embodiment, power supply 50 isconfigured to receive power from a wall outlet.

In some implementations control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some cases control programmabilityresides in the array driver 22. Those of skill in the art will recognizethat the above-described optimization may be implemented in any numberof hardware and/or software components and in various configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. In FIG.7B, the moveable reflective layer 14 is attached to supports at thecorners only, on tethers 32. In FIG. 7C, the moveable reflective layer14 is suspended from a deformable layer 34, which may comprise aflexible metal. The deformable layer 34 connects, directly orindirectly, to the substrate 20 around the perimeter of the deformablelayer 34. These connections are herein referred to as support posts. Theembodiment illustrated in FIG. 7D has support post plugs 42 upon whichthe deformable layer 34 rests. The movable reflective layer 14 remainssuspended over the cavity, as in FIGS. 7A-7C, but the deformable layer34 does not form the support posts by filling holes between thedeformable layer 34 and the optical stack 16. Rather, the support postsare formed of a planarization material, which is used to form supportpost plugs 42. The embodiment illustrated in FIG. 7E is based on theembodiment shown in FIG. 7D, but may also be adapted to work with any ofthe embodiments illustrated in FIGS. 7A-7C as well as additionalembodiments not shown. In the embodiment shown in FIG. 7E, an extralayer of metal or other conductive material has been used to form a busstructure 44. This allows signal routing along the back of theinterferometric modulators, eliminating a number of electrodes that mayotherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIG. 7, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields the portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34. This allows theshielded areas to be configured and operated upon without negativelyaffecting the image quality. Such shielding allows the bus structure 44in FIG. 7E, which provides the ability to separate the opticalproperties of the modulator from the electromechanical properties of themodulator, such as addressing and the movements that result from thataddressing. This separable modulator architecture allows the structuraldesign and materials used for the electromechanical aspects and theoptical aspects of the modulator to be selected and to functionindependently of each other. Moreover, the embodiments shown in FIGS.7C-7E have additional benefits deriving from the decoupling of theoptical properties of the reflective layer 14 from its mechanicalproperties, which are carried out by the deformable layer 34. Thisallows the structural design and materials used for the reflective layer14 to be optimized with respect to the optical properties, and thestructural design and materials used for the deformable layer 34 to beoptimized with respect to desired mechanical properties.

FIG. 8 schematically illustrates a portion of an exemplaryinterferometric modulator array 501. The interferometric modulator array501 is formed on a substrate 500, which is transparent for apredetermined light spectrum and has a bottom surface 400. Although notlimited thereto, the substrate 500 is preferably made of glass. A singlelayer or stack of layers 502 is formed over the substrate 500. Thesingle layer 502 or at least one sub-layer (not shown) of the stack oflayers 502 is made of a conductive material. The layer 502 or asub-layer serves as a partial mirror as it both reflects and transmitssome of the light incident thereto. For the sake of convenience, theterm “stationary layer 502” is used to refer to the single layer orstack of layers 502 unless the specific terms are used. Deformablelayers 506 are located over the stationary layer 502. Support posts 504are formed between the substrate 500 and the layers 506, separating thedeformable layers 506 from the substrate 500 and the stationary layer502. The deformable layers 506 lie in a generally parallel plane to thatof the stationary layer 502. The surface of the deformable layers 506facing the stationary layer 502 is highly reflective of thepredetermined light spectrum and serves as a full mirror.

This interferometric modulator array 501 is operated by applying or notapplying an electric potential difference between the conductive portionof the stationary layer 502 and the deformable layers 506. By applying acertain electric potential difference between them, for example 7 volts,the deformable layer 506 is driven to deform toward and contact thestationary layer 502 as in the case of the interferometric modulator 501b. In this driven state, the interferometric modulator 501 b is, forexample, in an induced absorption mode, in which most of the lightincident to the substrate 500 is absorbed by the interferometricmodulator 501 b. If the interferometric modulator 501 b is designed tooperate in the visible light spectrum, the bottom surface 400 of thesubstrate 500 corresponding to the area of interferometric modulator 501b turns to black at the driven state.

The interferometric modulator 501 a, on the other hand, is illustratedin the configuration produced when no voltage is applied between thedeformable layer 506 and the stationary layer 502. This configuration isreferred to as “the undriven state.” In this state, the deformable layer506 is maintained separate from the stationary layer 502, forming aspace 499 referred to as an “interferometric cavity” between them. Moreaccurately, the interferometric cavity 499 is defined as the distancebetween the reflective surface of the deformable layer 506 and thepartial mirror surface of the stationary layer 502. Light that isincident to the interferometric modulator 501 a through the substrate500 is interferometrically modulated via the cavity 499. Depending onthe depth of the cavity 499, which is the distance between the partialmirror surface of the stationary layer 502 and the full mirror surfaceof the deformable layer 506, the interferometric modulation selects acertain wavelength of the light, which is reflected from the bottomsurface 400 of the substrate 500. If the selected wavelength of thelight is visible, the bottom surface 400 of the substrate 500 displays avisible light corresponding to the wavelength. One of ordinary skill inthe art will well appreciate the interferometric modulation produced inthe interferometric modulator 501.

FIG. 9A is a cross-sectional view of the interferometric modulator 501of FIG. 8 taken along lines 9A-9A. FIG. 9A illustrates additionalinterferometric modulators 501 c-501 e arranged in the lateral directionof the interferometric modulator 501 b. In the illustrated embodiment,the stationary layer 502 is comprised of three sub-layers, for example,including a dielectric layer 413, a mirror layer 415 and a conductorlayer 417. As illustrated, the deformable layer 506 is laterally spacedby the posts 504 and substantially parallel with the stationary layer502, creating an interferometric cavity 418 between them. Although notillustrated, additional layers may be formed over the deformable layer506. The overall micro-structure formed over the substrate 500constitutes an array of interferometric modulators or array 411. Theinterferometric modulator 501 c is illustrated in an undriven state,which generally reflects a certain light through the substrate 500depending upon the depth of the interferometric cavity 418. Again, thisdepth determines the wavelength of light reflected on the surface 400.The interferometric modulator 501 b is illustrated in a driven state,which generally reflects no light on the surface 400. The operation ofthe interferometric modulators 501 b and 501 c will be well appreciatedby one of ordinary skill in the art.

FIG. 9B illustrates the micro-construction of another embodiment of theinterferometric modulator 501. In this embodiment, the deformable layer506 is connected to a mirror 419, which is located between thedeformable layer 506 and the stationary layer 502. All of the otherfeatures are the same as in the embodiment of FIG. 9A. In oneembodiment, the mirror 419 is substantially rigid and has a highlyreflective surface facing the stationary layer 502. The deformable layer506 functions to control the location of the mirror 419 with respect tothe stationary layer 502, and the rigid mirror 419 does not experienceany significant bending or deformation in this process. In thisembodiment, the interferometric cavity 418 is defined by the spacebetween the mirror 419 and the stationary layer 502, and more accuratelythe mirror layer 415. The interferometric modulator 501 c is illustratedin an undriven state, while the interferometric modulator 501 b isillustrated in a driven state.

In the embodiments illustrated in FIGS. 9A and 9B, the stationary layer502 may be formed by a single layer functioning as both a conductor anda mirror. Alternatively, the stationary layer 502 may be formed of twolayers, for example the pair of a mirror layer and a conductive layer,the pair of a dielectric layer and a bi-functional layer of electrodeand mirror. Further, in other embodiments, one or more additional layersmay be formed over the stationary layer 502 or in between the layers413, 415 and 417. Also, although not illustrated, the deformable layer506 or the mirror 419 of the embodiments of FIGS. 9A and 9B may have alaminated construction. For example, a dielectric layer may be formed ona surface of the deformable layer 506 (FIG. 9A) or the mirror 419 (FIG.9B), particularly the surface facing the stationary layer 502. Thedielectric layer on the deformable layer 506 (FIG. 9A) or the mirror 419(FIG. 9B) may be useful when the stationary layer 502 has theconstruction that does not include the dielectric layer 413. One ofordinary skill in the art will appreciate the formation of various filmsor layers making the stationary layer 502 and/or the additional layersthat can be formed on the deformable layer 506 or mirror 419.

In a typical construction, as illustrated in FIGS. 8, 9A and 9B, thedeformable layer 506 or the mirror 419 may physically contact thestationary layer 502 during its operation, particularly when theinterferometric modulator 501 is in its driven state. Physical contactor interaction between the two layers may cause some adverse results,particularly if it is between the surfaces defining the interferometriccavity, which are mirror surfaces of the stationary layer 502 and thedeformable layer 506 (or mirror 419). The dielectric layer 413 over themirror layer 415 is provided to minimize or reduce the mechanical and/orelectrical interactions between the surfaces forming the interferometriccavity. For the same reason, a dielectric layer (not shown) can beformed on the surface of the deformable layer 506 or the mirror 419.However, repeated changes between the driven and undriven states caneventually result in degradation of such dielectric layers mechanicallyand/or electrically.

Also, the dielectric layers may contain some charges in them due to, notlimited to, imperfection of the manufacturing processes. The charges inthe dielectric layers may create attractive forces between thedeformable layer 506 (or mirror 419) and the stationary layer 502. Someadditional force may be needed to separate the deformable layer 506 (orthe mirror 419) from the stationary layer 502 when a unit of theinterferometric modulator 501 is operating from its driven state toundriven state. Also, when the dielectric layer 413 contacts thedeformable layer 506 (or the mirror 419), there may be some other formof attractive force between the materials of the two contacting layers.Furthermore, even in an embodiment where the stationary layer 502 doesnot contact the deformable layer 506 (or the mirror 419) in the drivenstate, the gap between them is generally very small, for example, in theorder of 200 Å (20 nm). In certain conditions, moisture from thesurrounding environment may condense in the small gap and form a liquidlayer. To separate the layers in that condition, additional forceovercoming the surface tension of the liquid layer is needed.

The degradation of the dielectric layer(s) and the need for additionalforces may be overcome by various techniques and features of embodimentsdescribed herein, which include use of components such as landing pads,bumps and springs. Although introduced in light of the degradation ofthe dielectric layer and the associated need for the additional force,the below-described technical features may be used in any constructionsof the interferometric modulator utilizing the MEMS technology withoutsuch degradation or need of additional force. For the sake ofsimplicity, the below-described embodiments of the interferometricmodulators have the general architecture illustrated in FIGS. 8 and 9A.However, all of the features can be applied to any other architecture ofthe interferometric modulators, including the embodiment illustrated inFIG. 9B.

An embodiment provides an interferometric modulator, comprising: a firstlayer comprising a first reflective planar portion; a second layercomprising a second reflective planar portion located substantiallyparallel to the first reflective planar portion, the second layermovable between a first position and a second position, the firstposition being a first distance from the first layer, the secondposition being a second distance from the first layer, the seconddistance being greater than the first distance; and a member having asurface located between the first layer and the second layer, the memberdefining one or more gap regions between the first layer and the secondlayer when the second layer is in the first position, wherein the secondlayer in the one or more gap regions does not contact either the firstlayer or the member. Various aspects of this embodiment are described ingreater detail below.

Landing Pads

FIGS. 10A and 10B illustrate an embodiment of the interferometricmodulator 301 which includes landing pads 513. In the illustratedembodiment, the landing pads 513 extend from the substrate 500 throughthe stationary layer 502 beyond the top surface of the stationary layer502. Accordingly, when the interferometric modulator 301 is driven fromits undriven state (FIG. 10A) to the driven state (FIG. 10B), travel ofthe deformable layer 506 is interrupted by the landing pads 513, whichoperate to prevent further travel of the deformable layer 506 toward thestationary layer 502, and thus to prevent the physical contact betweenthose layers 502 and 506, and to maintain a desired separation distancebetween the layers 506 and 502. As discussed above with reference toFIGS. 9A and 9B, the stationary layer 502 can be formed of a singlelayer or multiple layers. Also, the stationary layer 502 may or may notinclude a dielectric layer 413. It will be recognized that the landingpads 513 are examples of members having a surface 514 located betweenthe deformable layer 506 and the stationary layer 502. The landing pads513 define a gap region 418 a between the deformable layer 506 and thestationary layer 502 when the interferometric modulator 301 is in thedriven state (FIG. 10B). The deformable layer 506 in the gap region 418a does not contact either the stationary layer 502 or the landing pads513.

In another embodiment as illustrated in FIG. 10C, the landing pads 513may be formed on the top surface of the stationary layer 502. In anotherembodiment as illustrated in FIG. 10D, the landing pads 513 may extendfrom a sub-layer 415 of the stationary layer 502 through one or moreother sub-layers 413. In still another embodiment as illustrated in FIG.10E, the landing pads 513 may be integrally formed with the substrate500 and extend through the stationary layer 502. In a furtherembodiment, as illustrated in FIG. 10F, the landing pad 513 may extendfrom below the interface between the substrate 500 and the stationarylayer 502 and through both the substrate 500 and the stationary layer502.

In another embodiment as illustrated in FIG. 10G, the landing pads 513may be formed on the deformable layer 506 or mirror 419 (not shown). Inother embodiments as illustrated in FIGS. 10H and 10I, the landing pads513 may be formed on both the deformable layer 506 and the stationarylayer 502. In the foregoing embodiments where one or more landing pads513 are formed on the deformable layer 506, although not illustrated,the landing pads 513 may extend from various sub-layers, if any, of thedeformable layer 506, as the landing pads 513 may extend from varioussub-layers of the stationary layer 502 or substrate 500 illustrated inFIGS. 10A-10F.

The landing pads 513 can be positioned in various locations on thestationary layer 502 or the deformable layer 506, or both within theinterferometric cavity 418. FIG. 10J is a top cross-sectional view ofthe embodiment of FIG. 10A taken along line 10J-10J (FIG. 10A). In theillustrated embodiment of FIG. 10J, for example, the landing pads 513are located generally on peripheral portions of the stationary layer 502and/or deformable layer 506 within the interferometric cavity 418.Optionally, the landing pads 513 are positioned on the portions of thestationary layer 502 and/or deformable layer 506 where the existence ofthe landing pads 513 would not affect the optical characteristics of theinterferometric modulator 301. In another embodiment (not illustrated),the landing pads 513 can be positioned on a central portion of thestationary layer 502 and/or deformable layer 506 within theinterferometric cavity 418. In still another embodiment (notillustrated), the landing pads 513 can be positioned on both the centraland peripheral portions of the stationary layer 502 and/or deformablelayer 506 within the interferometric cavity 418. In a further embodiment(not illustrated), the landing pads 513 can be located where thedeformable layer 506 first contacts the stationary layer 502.

Referring again to FIGS. 10A and 10G, it is seen that the landing pads513 extend beyond the surface of the stationary layer 502 (FIG. 10A) orthe deformable layer 506 (FIG. 10G) by a height indicated at 519. In oneembodiment, the landing pad height 519 is selected so as to preventphysical contact between the deformable layer 506 and the stationarylayer 502. In another embodiment, the height 519 is selected to not onlyprevent such contact, but to define the depth of the interferometriccavity 418 in the driven state of the interferometric modulator 301, andso as to enable production of the desired optical characteristics of theinterferometric modulator 301. In one embodiment, the landing pads 513are configured to precisely define the distance between the two layers506 and 502. Thus, the landing pads 513 can be used to control theminimal size of the interferometric cavity 418 with a high degree ofaccuracy and precision.

In one embodiment of interferometric modulator 301 for use as a displayelement, the interferometric cavity depth at the driven state is shortenough to absorb most, if not all, of the visible light. In anotherembodiment of interferometric modulator 301 for use as a displayelement, the interferometric cavity depth at the driven state reflects aselected visible wavelength of incident light. Since the interferometriccavity depth at the driven state is determined by the thickness ofvarious layers and/or structures positioned between the reflectivesurfaces of the layers 502 and 506, the height 519 of the landing pad513 is designed in view of the color to be displayed on the substrate500. In such display embodiments, the height 519 is, for example, fromabout 50 Å to about 1500 Å, and preferably from about 100 Å to about 300Å.

In one embodiment, the landing surface 514 of the landing pads 513 issubstantially planar, as illustrated in FIG. 10A. Also as in theembodiment illustrated in FIG. 10A, the landing surface 514 may besubstantially parallel to the surface of the deformable layer 506 or thestationary layer 502 that lands on the landing surface 514. In thisembodiment, the size of the landing surface 514 is from about 0.1 micronto about 25 microns, and preferably from about 3 microns to about 10microns. In another embodiment as illustrated in FIG. 10C, the landingsurface 514 of the landing pads 513 may be rough, bumpy or embossed. Inanother embodiment as illustrated in FIG. 10F, the landing surface 514of the landing pads 513 may be tilted from the plane parallel to thecounterpart surface landing on the landing surface 514. In still anotherembodiment as illustrated in FIG. 10D, the landing surface 514 may besubstantially round.

The landing pads 513 can be made from various materials, including, butnot limited to, a metal, an alloy, a dielectric material, and anelastomeric material. For example, such materials may include metalsincluding aluminum, semiconductors, oxides of metals or semiconductors,nitrides of metals or semiconductors, and oxynitrides of metals orsemiconductors. Preferably, the materials forming landing pads 513 arethose that substantially do not or only insignificantly affect theelectrical or optical characteristics of the interferometric modulator301.

In one embodiment, the landing pads 513 are optically transparent forthe light spectrum which the interferometric modulator 301 can select.Optionally, in the case where the light spectrum includes visible light,the transparent material that can be used for the landing pads 513includes, for example, oxides of metals or semiconductors, nitrides ofmetals or semiconductors, and oxynitrides of metals or semiconductors.In another embodiment, the landing pads 513 may be made of a materialthat absorbs the light spectrum which the interferometric modulator 301can select. In another embodiment, the landing pads 513 may be coveredwith the light absorbing material. Optionally, in the case where thelight spectrum includes visible light, the light absorbing material thatcan be used for the landing pads 513 includes, for example, polymericmaterials or metals, such as chrome, nickel, titanium, molybdenum, etc.In still another embodiment, the landing pads 513 may be made of amaterial that reflects the light spectrum which the interferometricmodulator 301 can select. In still another embodiment, the landing pads513 may be covered with the light reflecting material. Optionally, inthe case where the light spectrum includes visible light, the lightreflecting material that can be used for the landing pads 513 includes,for example, polymeric materials or metals, such as silver, aluminum,gold, platinum, etc.

In a unit of the interferometric modulator 301, multiple landing pads513 can be used. Thus, 2, 3, 4, 5, 6 or more landing pads 513 can befabricated to provide the landing surfaces of the layers of theinterferometric modulator 301. Preferably, the multiple landing pads 513have substantially the same heights 519. Optionally, the multiplelanding pads 513 are arranged as remote as possible from one another onthe stationary layer 502 or the deformable layer 506. In one embodiment,a single landing pad 513 per unit of the interferometric modulator 301can be used.

The landing pads 513 may be positioned in any cross-sectional shapelying in a plane parallel to the stationary layer 502. In the embodimentillustrated FIG. 10J, the cross-sectional shape of the landing pads 513is substantially circular, oval, rectangular and pentagonal, althoughnot limited thereto.

The landing pads 513 can be fabricated in various configurations andmade of various compounds as discussed above, utilizing the presentlyexisting techniques of depositing and selectively etching a material. Inone embodiment, the landing pads 513 can also be created fromdeformations of the layers of the interferometric modulator 301. Inanother embodiment, the landing pads 513 can be created usingconventional semiconductor manufacturing techniques.

MEMS devices often comprise an array of individual elements activated byapplication of a voltage potential. The elements may comprise manydifferent types of structures, including mirrors, switches, gears,motors, etc. The application of the voltage potential may be done byapplying the potential directly to the structure, or by manipulation ofelectrical or magnetic fields around the structure. For example, anelement may be activated by electrostatic attraction between the elementand another structure to which the voltage is applied. For purposes ofthis discussion, the structure to which the voltage is applied will bereferred to as an electrode.

In this type of device, there is generally a gap between the element andthe electrode. This gap may give rise to capacitive charge between theelement and the electrode. For most MEMS devices with this type ofstructure, the performance of the device will be improved by loweringthe capacitance in the gap. This reduction of capacitance produces morepredictable performance, and there is a lowered risk of capacitivedischarge, which can damage the element or the neighboring elements.

In a bi-chrome display, such as a display that switches between blackand white, one interferometric modulator element might correspond to onepixel. In a color display, three or more interferometric modulatorelements may make up each pixel, e.g., one each for red, green and blue.The individual interferometric modulator elements are controlledseparately to produce the desired pixel reflectivity. Typically, avoltage is applied to the movable wall, or element, of the cavity,causing it to be electrostatically attracted to the other electrode,resulting in a change in the color of the pixel seen by the viewer.

The interferometric modulator is merely one type of an active MEMSdevice that has an element separated from an electrode, where theelectrode is used to activate the device. Another example may be a MEMSswitch. These devices may suffer from high capacitance that may affecttheir operation. If a device has high capacitance in the mechanicallyrelaxed state, it may take longer for the attractive charge to activatethe device, slowing the device response time.

The capacitance of the device can be approximated by the capacitance ofan idealized parallel-plate capacitor, given by C=∈A/d, where ∈ is theelectrical permittivity of the material between the movable wall and theelectrode, A is the surface area of the electrode, and d is the gapdistance between the movable wall and the electrode. The electricalpermittivity of a material is equal to the dielectric constant K of thematerial multiplied by the electrical permittivity ∈₀ of vacuum. Invarious embodiments, the capacitance between the movable wall and theelectrode is reduced by increasing the size of the gap between theelectrode and the movable wall and/or by lowering the dielectricconstant of the material within the gap (that is, by decreasing ∈ in theabove equation). For example, the gap can comprise a material with a lowdielectric constant, such as a gas or a mixture of gases (e.g., air).This use of a material within the gap with a reduced dielectric constanthas the effect of lowering the capacitive charging of the dielectricsurface, thereby lowering the capacitance.

An embodiment of a processing flow for a MEMS device is shown in FIG.11. In that embodiment, an electrode is formed on a substrate at step150. A multilayer dielectric stack is deposited at step 152, andpatterned at step 154. Portions of the multilayer dielectric stack,e.g., a thin oxide stop layer, are removed at step 156. The MEMS devicethen undergoes its appropriate processing at step 158, where theprocessing includes the use of a sacrificial layer to form the gap. Thesacrificial layer, and portions of the multilayer dielectric stack notunder the oxide stops, are removed at step 160. In another embodiment, agraded dielectric material is deposited at step 152 instead of themultilayer dielectric stack. The remainder of the process illustrated inFIG. 11 continues in a similar manner, including removing upper portionsof the graded dielectric material at step 156, and removing lowerportions of the graded dielectric material at step 160, along with thesacrificial layer.

An embodiment of an interferometric modulator having a multilayerdielectric stack is shown in FIG. 12. In this embodiment the portions ofthe dielectric stack 513 not removed appear across the device 140,rather than just under the support posts 18. The process of forming theoxide stops can be modified as desired to leave portions of thedielectric stack wherever desired.

FIGS. 13-19 illustrate an embodiment of a process for the fabrication ofan interferometric modulator that includes landing pads 513, usingconventional semiconductor manufacturing techniques such asphotolithography, deposition, masking, etching (e.g., dry methods suchas plasma etch and wet methods), etc. Deposition includes “dry” methodssuch as chemical vapor deposition (CVD, including plasma-enhanced CVDand thermal CVD) and sputter coating, and wet methods such as spincoating. FIG. 13 illustrates the formation of a stationary layer 502,which can be a single layer structure or multiple sub-layer structure asdescribed above. In a single layer structure where the layer 502functions as both electrode and mirror, the layer 502 is formed bydeposition of an electrode material 410 on the substrate 500 andsubsequent patterning and etching. The electrode material 410 isconductive and may be a metal or a semiconductor (such as silicon) dopedto have the desired conductivity. In one embodiment (not shown in FIG.13), the electrode layer 410 (and the corresponding first electrode 502)is a multilayer structure comprising a transparent conductor (such asindium tin oxide) and a primary mirror (such as chromium).

FIG. 14 illustrates the formation of a dielectric layer 540 on thesubstrate 500 and the stationary layer 502 by deposition, preferably byCVD. The lower or “bulk” portion 550 of the dielectric layer 540 neednot be a dielectric material and is preferably a material that may beremoved in a later etching step, and thus may be molybdenum, asilicon-containing materials (e.g., silicon, silicon nitride, siliconoxide, etc.), tungsten, or titanium, preferably silicon oxide. The upperor “stop” portion 560 of the dielectric layer 540 is preferably amaterial that is more resistant to a later etching step than the bulkportion 550, and may be a metal (e.g., titanium, aluminum, silver,chromium) or a dielectric material, preferably a metal oxide, e.g., analuminum oxide. Aluminum oxide may be deposited directly or bydeposition of an aluminum layer followed by oxidation. The upper andlower portions 550, 560 of the dielectric layer 540 may be composed ofthe same material or may be different materials. Additional layers,e.g., intermediate layers, may also be formed over the stationary layer502. For example, in an embodiment (not shown), an intermediate layer isformed over at least a portion of the stationary layer 502, and thedielectric layer 540 is formed over the intermediate layer and over thestationary layer 502 underlying the intermediate layer. Suchintermediate layer(s) formed between the stationary layer 502 and thedielectric layer 540 may be utilized for various purposes. For example,the intermediate layer may be an optical layer, a barrier layer and/or anon-conductive layer (such as a second dielectric layer). In anembodiment, in any particular dielectric layer 540, at least one of theportions 550, 560 is an electrical insulator.

The upper portion 560 may be thinner or thicker than the lower portion550. For example, in one embodiment the upper portion 560 may have athickness in the range of about 50 Å to about 500 Å, and the lowerportion 550 may have a thickness in the range of about 200 Å to about3000 Å. As described in greater detail below, the upper or “stop”portion 560 may serve as an etch barrier (e.g., functioning in a mannersomewhat analogous to a photomask) during a later process step, and apart of the lower portion 550 may serve as a “sacrificial” layer that isremoved. In this embodiment, the upper portion 560 is more resistant toremoval (e.g. by etching) than the lower portion 550. In a particularembodiment, the upper portion 560 is aluminum oxide and the lowerportion 550 is silicon oxide. The upper and lower portions 550, 560 neednot be distinct layers and thus the dielectric layer 540 may be a gradedlayer. For example, the dielectric layer 540 may be compositionallygraded so that the composition varies as a function of position (e.g.,as a function of vertical position in FIG. 14) within the dielectriclayer. For example, the dielectric layer 540 may be a graded siliconnitride layer in which the relative amounts of silicon and nitrogen varyon going from the upper surface 420, 421 to the interface 422, 423 withthe first electrode layer 502 and the substrate 500. In one embodiment,for example, the graded silicon nitride layer is enriched in silicon atthe interface 421 with the first electrode 502 relative to the overallcomposition of the graded silicon nitride. In another embodiment, thedielectric layer 540 may be a graded silicon oxide layer in which therelative amounts of silicon and oxygen vary on going from the uppersurface 420, 421 to the interface 422, 423 with the first electrodelayer 502 and the substrate 500. In one embodiment, for example, thegraded silicon oxide layer is enriched in silicon at the interface 421with the first electrode 502 relative to the overall composition of thegraded silicon oxide.

FIG. 15 shows that parts of the upper portion 560 are then removed toform “stops” 565 by masking the upper portion 560 with a photomask 610,then etching to selectively remove the exposed part of the upper portion560 of the dielectric layer 540 to form a variable thickness dielectriclayer 570 as illustrated in FIG. 16. The etching is carried out toexpose part of the lower portion 550 of the dielectric layer 540. Theetching is controlled so that a substantial portion of the lower portion550 of the dielectric layer 540 remains. For example, a small part ofthe lower portion 550 may be removed during etching, but most of thelower portion 550 preferably remains until it is removed duringsubsequent processing as described below, thereby increasing theunevenness of the dielectric layer and increasing the averagepeak-to-valley surface variation of the dielectric layer.

The fabrication process continues as illustrated in FIG. 17, includingformation of a sacrificial layer 710 (which is later removed to form theinterferometric cavity 418) by deposition, patterning and etching;formation (and optional planarization) of the posts 504; and formationof the deformable layer 506 by deposition, patterning and etching.Sacrificial layer 710 is preferably molybdenum. In an embodiment, thedeformable layer 506 is an upper electrode. Because these steps arecarried out over variable thickness dielectric layer 570, the interfacebetween sacrificial layer 710 and deformable layer 506 may not becompletely flat. For example, in the illustrated embodiment, the lowersurface contour 741, 742 of the deformable layer 506 tends tosubstantially parallel the contours of the layers beneath it, e.g., thesteps in the variable thickness dielectric layer 570. However, thoseskilled in the art will understand that variable thickness dielectriclayer 570 may have a thickness of only 100 Å, and thus FIG. 17 (not toscale) may exaggerate the undulations in the lower contour 741, 742.

FIG. 18 illustrates etching with an etchant to remove the “sacrificial”layers, sacrificial layer 710 and the exposed part of the lower portion550. As the etchant, XeF₂, F₂ or HF may be used alone or in combination.The upper or “stop” portion 565 substantially protects the part of thelower portion 550 that is beneath it from being removed by etching,functioning in a manner somewhat analogous to a photomask. The resultinginterferometric modulator 1800 illustrated in FIG. 19 includes theinterferometric cavity 418, a portion 910 of the stationary layer 502that is not covered by a variable thickness dielectric layer 920(comprising the upper variable thickness dielectric layer 565 and avariable thickness lower portion 925). The lower portion 550 need not becompletely removed by etching, and thus part of the lower portion 550may remain over the stationary layer 502, preferably where thestationary layer 502 is a single conductor layer.

This invention is not limited by theory, but it is believed that XeF₂serves as a convenient source of F₂ gas. Other etchants such as F₂ andHF may be used in place of or in addition to XeF₂. In an embodiment, theetchant removes the lower portion 550 at an etch rate that is higherthan an etch rate for removing the upper portion 565. Thus, in anembodiment, the difference in average thickness variation between thelower surface contour 741, 742 of the deformable layer 506 and the uppercontour of the variable thickness dielectric layer 570 tends to increaseas etching proceeds, e.g., as the variable thickness dielectric layer570 is etched to form the variable thickness dielectric layer 920.

The variable thickness dielectric layer 920 comprises landing pads 513.The landing pads 513 project upward from the stationary layer 502 andsubstantially prevent contact between the stationary layer 502 and thedeformable layer 506, during both the driven and undriven states. Thevariable thickness dielectric layer may be a discontinuous layer, e.g.,as illustrated by dielectric layer 920 in FIG. 19, or may be acontinuous layer in which the thickness variation is manifested as peaksand valleys on the surface of the layer.

It will be appreciated by those skilled in the art that the variablethickness dielectric layer 920 may comprise multiple columns ofdielectric material that project upward from the bottom electrode andsubstantially prevent contact between the first and second electrode,during both the driven and undriven states, e.g., as illustrated in FIG.12. Thus, the remaining surface area of the bottom electrode (e.g., thesurface portion 910 not covered by such a column) need not be coated orcovered by an insulating layer. A substantial improvement in capacitanceis thus obtained, because the dielectric constant of air (about 1) islower than that of insulating materials such as metal oxides disclosedin U.S. Pat. No. 5,835,255. The variable thickness dielectric layer maybe a discontinuous layer, e.g., as illustrated by dielectric layer 920in FIG. 15, or may be a continuous layer in which the thicknessvariation is manifested as peaks and valleys on the surface of thelayer. In either case, the distance between the top of the landing pad513, for example, and the bottom of the valley or gap 910, for example,is preferably about 50 Å or greater, more preferably in the range ofabout 100 Å to about 3,000 Å.

Those skilled in the art will appreciate that, in the illustratedembodiment of FIG. 18, the upper or “stop” portion 565 that is patternedabove the lower or “bulk” portion 550 prevents the bulk layer from beingcompletely etched away by the XeF₂ (similar to any masking step used topattern previous layers). The areas of the bulk layer 550 that are notprotected by the stop portion 565 form a sacrificial portion that islater removed, and the portions of the bulk material 925 below the stop565 remain, forming a variable thickness dielectric layer 920(comprising an upper layer 565 and a lower layer 925), e.g., comprisingone or more islands or columns of multilayer dielectric material thatsubstantially prevent contact between the first and second electrodes.Although the lower contour 741, 742 of the underside of the deformablelayer 506 illustrated in FIG. 18 tends to substantially parallel theupper contour of the variable thickness dielectric layer 570, it doesnot substantially parallel the upper contour of the variable thicknessdielectric layer 920 illustrated in FIG. 19 because etching removes atleast a part of the lower portion 550 that is not protected by the upperportion 565 of the variable thickness dielectric layer 570. This etchingto remove the exposed part of the lower portion 550 creates extra spacebetween the lower contour 742 of the deformable layer 506 and thesurface portion 910 of the stationary layer 502.

FIG. 19 illustrates an actuated interferometric modulator 1801. Duringactuation, the lower contour 741 of the actuated deformable layer 506 amay contact the top of the stops 565, e.g. at the landing pads 513 inthe illustrated embodiment, thereby creating regions in which the lowercontour 741 of the deformable layer 506 is spaced from the surfaceportion 910 of the lower electrode 502. These regions include a lowdielectric constant gap 418 a between the lower contour 742 of theactuated deformable layer 506 a and the surface portion 910 of thestationary layer 502. Thus, as illustrated in FIG. 19, the profile ofthe underside of the actuated deformable layer 506 a is different fromthe profile of the upper side of the variable thickness dielectric layer920, so that the low dielectric constant gap 418 a exists between theactuated deformable layer 506 a and the stationary layer 502 duringoperation. Thus, the lower surface of the deformable layer 506 has asurface profile variation 741, 742 that is less than a surface profilevariation of the variable thickness dielectric layer 920. In certainembodiments, the surface profile variation is equal to the averagepeak-to-valley surface profile variation. The average peak-to-valleysurface profile variation of the lower surface of the upper electrodemay be in the range of about 50 Å to about 200 Å. The averagepeak-to-valley surface profile variation of the variable thicknessdielectric layer may be in the range of about 200 Å to about 1000 Å.Average peak-to-valley surface profile variation may be determined byscanning electron microscopy and/or atomic force microscopy. In certainembodiments, the average peak-to-valley surface profile variation is thedifference between the average peak heights and the average valleydepths of the layer over a selected area.

It will be recognized that the landing pads 513 are examples of membershaving an upper surface located between the deformable layer 506 and thestationary layer 502. The landing pads 513 define a gap region 418 abetween the deformable layer 506 a and the stationary layer 502 when theinterferometric modulator 1801 is in the driven state (FIG. 19). Thelower surface contour 742 of the deformable layer 506 a in the gapregion 418 a does not contact either the stationary layer 502 or thelanding pads 513.

FIG. 20 illustrates another embodiment in which the sacrificial layer isplanarized before deposition of the upper electrode. The structure 1900illustrated in FIG. 20 may be formed from the structure 1600 illustratedin FIG. 17 by planarizing the sacrificial layer 710 to produce arelatively planar surface 746. In an alternative embodiment (notillustrated), the relatively planar surface is formed by depositing aplanarization layer over the sacrificial layer 710, instead of or inaddition to planarizing the sacrificial layer 710. A deformable layer506 is then formed over the surface 746 as illustrated in FIG. 16. In anembodiment, the deformable layer 506 is an upper electrode. Thesacrificial layer 710 may then be removed to form a gap 418 asillustrated in FIG. 21 in a manner generally similar to that illustratedin FIG. 18. Removal of the part of the lower portion 550 that is notprotected by the upper portion 565 of the variable thickness dielectriclayer 570 (as illustrated in FIG. 18) is optional for the configurationillustrated in FIGS. 20-21 because the lower contour 747 of thedeformable layer 506 is relatively planar. Thus, the profile of theunderside of the deformable layer 506 is different from the profile ofthe upper side of the variable thickness dielectric layer 570(regardless of whether the part of the lower portion 550 that is notprotected by the upper portion 565 of the variable thickness dielectriclayer 570 is removed or not) so that a low dielectric constant gap(s)exists between the upper deformable layer 506 and lower stationary layer502 during operation. Thus, the lower contour 747 of the deformablelayer 506 has a surface profile variation that is less than a surfaceprofile variation of the variable thickness dielectric layer 570.

In the illustrated embodiments, the variable thickness dielectric layer920 is formed over the stationary layer 502 (in this context, “over”refers to the relative location for the orientation illustrated in FIG.19). A variable thickness dielectric layer may be formed elsewhere inthe cavity 418, e.g., under the deformable layer 506. Thus, for example,a variable thickness dielectric layer may be formed on the firstelectrode and/or on the second electrode of an interferometricmodulator. Those skilled in the art will also appreciate that aninterferometric modulator may contain three or more electrodes, and thusmay contain two or more variable thickness dielectric layers, e.g., avariable thickness dielectric layer between each of the electrodes.

In the illustrated embodiment, portions of the cavity may contain a lowdielectric constant material, e.g., some or all of the interior walls ofthe cavity 418 may optionally be coated or covered with a low dielectricconstant material. For example, after etching to form theinterferometric modulator illustrated in FIG. 19, a layer of lowdielectric constant material (not shown) may be formed on the surfaceportion 910 of the stationary layer 502. Preferably, any such layer oflow dielectric constant material is relatively thin, such that a gapremains between the top electrode and the low dielectric constantmaterial during both the driven and undriven states. Other interiorwalls of the cavity 418 that may coated with a low dielectric constantmaterial include the deformable layer 506 (which may be an upperelectrode) and the variable thickness dielectric layer 565.

Silicon dioxide has a dielectric constant of approximately 3.8.Preferred low dielectric constant materials have a dielectric constantless than that of silicon oxide, i.e., less than 3.8. Exemplarymaterials compatible with embodiments described herein include, but arenot limited to, porous dielectric materials (e.g., aerogels) andmodified silicon oxides. See, e.g., U.S. Pat. Nos. 6,171,945 and6,660,656, both of which describe low dielectric constant materials andmethods for making them which are compatible with embodiments describedherein. Preferred low dielectric constant materials have a dielectricconstant of about 3.3 or less, more preferably about 3.0 or less, andmost preferably about 2.0 or less.

In another embodiment (not illustrated), a variable thickness dielectriclayer is formed by depositing a dielectric layer having a relativelyuniform thickness on the first and/or second electrodes (e.g., over thestationary layer 502 as shown in FIG. 13), then continuing thefabrication process as shown in FIGS. 14-16 but without the masking stepshown in FIG. 15. Then, during subsequent etching (e.g., as illustratedin FIGS. 18-19), the flow of the etchant is controlled so that thedielectric layer having a relatively uniform thickness is etched to agreater extent in some areas than others, resulting in a variablethickness dielectric layer.

It will be appreciated by those skilled in the art that a variablethickness dielectric layer, e.g., comprising multiple columns ofdielectric material that project upward from the bottom electrode, mayalso reduce damping of the interferometric modulator during operation,and thus may provide increased device switching speed by facilitatingescape of the damping medium (e.g., air) from the cavity. It will alsobe appreciated that the variable thickness dielectric layer has areduced dielectric constant as compared to a comparable uniformthickness dielectric layer of the same overall thickness as the variablethickness dielectric layer. The reduced dielectric constant mayadvantageously reduce the RC time constant of the interferometric deviceinto which it is incorporated, based on the relationshiptime=resistance×capacitance, thus increasing device switching speed.Certain embodiments provide an interferometric modulator made by aprocess described herein, wherein the interferometric modulatorcomprises a variable thickness dielectric layer. Such an interferometricmodulator may have a lower capacitance than a comparable interferometricmodulator having a uniform thickness dielectric layer in place of thevariable thickness dielectric layer. Such an interferometric modulatormay also have increased performance (e.g., increased switching speedresulting from reduced damping and/or from a reduced RC time constant)than a comparable interferometric modulator having a uniform thicknessdielectric layer in place of the variable thickness dielectric layer. Itwill also be appreciated that use of a variable thickness dielectriclayer as described herein may result in reduced contact area betweenmoving parts of the MEMS device, e.g., a reduced contact area betweenthe dielectric layer and the movable electrode. This reduction incontact area may result in increased mechanical reliability and/orreduced wear. Electrical reliability may also be improved by use of avariable thickness dielectric layer that results in reduced electricalcontact area with the moveable electrode. Such reduced electricalcontact area may result in reduced electrical charging of the dielectriclayer.

Bumps

FIGS. 22A and 22B illustrate an embodiment of an interferometricmodulator 401 that includes bumps 511. In the illustrated embodiment, aplurality of bumps 511 is formed on the top surface of the stationarylayer 502. Accordingly, when the interferometric modulator 401 is drivenfrom its undriven state (FIG. 22A) to the driven state (FIG. 22B), thedeformable layer 506 contacts the bumps 511, which act to prevent orminimize the physical contact between the deformable layer 506 and thestationary layer 502. Further, with the existence of the bumps, the areaof contact between the deformable layer 506 and the stationary layer 502can be reduced.

As discussed above with reference to FIGS. 9A and 9B, the stationarylayer 502 includes at least one conductive layer but can be formed of asingle layer or multiple layers. In any of the constructions of thestationary layer 502, the bumps 511 are preferably located on the topsurface of the stationary layer 502. In one embodiment, the top surfaceis made of a dielectric material and the bumps 511 are located on thedielectric surface. In another embodiment, the top surface of thestationary layer 502 is made of a conductive layer, and the bumps 511are located on the conductive surface.

In another embodiment as illustrated in FIG. 22C, the bumps 511 may belocated on the deformable layer 506 or mirror 419 (not shown). Again,the deformable layer 506 (or mirror 419) may include multiplesub-layers. In any of the constructions, the bumps 511 are preferablylocated on the surface of the deformable layer 506 (or mirror 419)facing the stationary layer 502. In another embodiment as illustrated inFIG. 22D, the bumps 511 may be located on both the deformable layer 506and the stationary layer 502.

The plurality of bumps 511 can be positioned in various locations on thestationary layer 502 and/or the deformable layer 506 within theinterferometric cavity 418. In one embodiment, the bumps 511 are locatedthroughout the surface of the stationary layer 502 and/or the deformablelayer 506. In another embodiment, the bumps 511 are located primarily ona central portion of the stationary layer 502 or the deformable layer506. In the area where the bumps 511 are located, the bumps 511 may beregularly, sporadically or randomly arranged on the surface of thestationary layer 502 or the deformable layer 506.

The bumps 511 may be fabricated in various shapes. In an embodiment asillustrated in FIG. 22E, the bumps 511 may not have a regular shape andmay comprise irregular protrusions from the stationary layer 502 or thedeformable layer 506. In other embodiments, the bumps 511 may have oneor more regular shapes as illustrated in FIGS. 22A-22D. In theembodiments of regularly shaped bumps, the bumps 511 may have a distalsurface 512 (FIG. 22A). In the illustrated embodiments, the distalsurface 512 is substantially planar and parallel to the counterpartsurface of the deformable layer 506 (or the stationary layer 502 in theembodiment of FIG. 22C or the counterpart bumps in the embodiment ofFIG. 22D). In another embodiment, the distal surface 512 may be planarbut tilted with reference to the counterpart surface (not illustrated).In still another embodiment, the distal surface 512 of the bumps 511 maybe round or rough (not illustrated).

The bumps 511 protrude from the stationary layer 502 or the deformablelayer 506 by a height indicated at 515 of FIG. 22A. The height 515 of abump 511 is defined as the distance between the distal end (distalsurface 512 in FIG. 22A) of the bump 511 and the surface from which thebump 511 protrudes. In some situations where the bumps are formed of thesame material as the underlying layer and are shaped irregularly, thereference surface may be difficult to determine. In such cases, theheight 515 of a bump 511 is the farthest distance between the distal endof the bump and the surface of the stationary layer 502 and/or thedeformable layer 506. In some embodiments, the bumps 511 havesubstantially the same height 515. In other embodiments, each of thebumps 511 may have a different height.

In one embodiment, the height 515 is selected so as to prevent physicalcontact between the deformable layer 506 and the stationary layer 502.In another embodiment, the height 515 is selected not only to preventsuch contact, but to define the depth of the interferometric cavity 418in the driven state of the interferometric modulator 401, so as toenable production of the desired optical characteristics of theinterferometric modulator 401. In the embodiments of interferometricmodulator 401 for use as a display element, the interferometric cavitydepth at the driven state is designed to be short enough to absorb most,if not all, of the visible light. Although not so limited, the height515 of the bumps 511 can be substantially smaller than the height 519 ofthe landing pads 513. The height 511 is from about 50 Å to about 500 Å,and preferably from about 100 Å to about 200 Å.

In a unit of the interferometric modulator 401, a number of bumps 511can be provided. As noted above, the bumps 511 are provided to preventthe stationary layer 502 and the deformable layer 506 from directlycontacting each other, and also to reduce the contact area of the twolayers 502 and 506. The number of the bumps 511 in a unit of theinterferometric modulator 401 is determined in view of the height 515thereof. For example, if the height 515 of the bump 511 is significantlylarge, only very few bumps 511 are necessary to effectively prevent thecontact between the stationary layer 502 and the deformable layer 506because it is unlikely that the deformable layer 506 in contact with thetall bumps 511 can also contact the stationary layer 502. On the otherhand, when the height 515 of the bumps 511 is small, more bumps 511 maybe needed.

The plurality of bumps 511 can be fabricated from various materials. Inone embodiment, the bumps 511 are made of a dielectric material. If thebumps 511 extend from a dielectric surface of the stationary layer 502or the deformable layer 506, the bumps 511 may be made of the samedielectric material. Alternatively, the bumps 511 may be formed ofanother dielectric material of the surface from which they extend. Inanother embodiment, the bumps 511 are made of a conductive material.Preferably, the materials used to form for the bumps 511 are those thatdo not significantly affect the electrical or optical characteristics ofthe interferometric modulator. For example, materials for the bumps 511may include oxides, nitrides and oxynitrides. Preferably, the bumps 511are substantially transparent to predetermined wavelengths of light.

The bumps 511 can be produced in a number of ways. In one embodiment,the bumps 511 are formed by the process described above for theproduction of landing pads 513. In one embodiment, a material isdeposited over the stationary layer 502 or the deformable layer 506, andthe material is etched to form the bumps 511 on the layer 502 or 506.The layer to be etched to form the bumps may comprise the same materialas the top or sole layer of the stationary layer 502 or layer 506. Forexample an exposed SiO₂ layer formed over the stationary layer 502 maybe etched with an etchant to produce a rough surface, thereby formingbumps 511. The etching process can be random, or the etching can befurther directed into particular shapes through the use of particularetching barriers. This can allow one to control the size and shape ofthe bumps and create patterns which may be optimized for reducing orpreventing the adverse impact created by contact of the deformable layer506 with the stationary layer 502.

Spring Clips

FIGS. 23A-23F illustrate embodiments of an interferometric modulator 501including spring clips 509. In typical constructions of theinterferometric modulator e.g., as illustrated in FIGS. 8, 9A and 9B,the deformable layer 506 has a tension in its deformed (driven) state501 b and has a tendency to return to its non-deformed (undriven) state501 c to reduce the tension. The tension of the deformable layer 506 inits deformed state creates a mechanical restoring force that is exertedon that layer 506 in the direction away from the stationary layer 502.The deformable layer 506 returns from its deformed state 501 b to theundeformed state 501 c when the mechanical restoring force overcomes theattractive force created by the electrical potential applied between thedeformable layer 506 and the stationary layer 502. As will be describedbelow in detail, the spring clips 509 are provided to help the recoveryof the deformable layer 506 from its driven state to the undriven stateby applying an additional element of force onto the deformable layer 506in the direction away from the stationary layer 502. When combined withthe mechanical restoring force of the deformable layer 502, theadditional element of force can increase the likelihood and/or speed ofthe return of the deformable layer 506 to the driven state when thereturn is desired.

In the illustrated embodiment of FIGS. 23A and 23B, the spring clips 509are provided on the stationary layer 502 of the interferometricmodulator 501. Referring to FIG. 23A which illustrates the undrivenstate, a portion of the spring clip 509 is located on the top surface ofthe stationary layer 502, and the tip 510 of the spring clip 509 is bentso as to extend into the interferometric cavity 418 toward thedeformable layer 506. In this undriven state, the spring clips 509 arein their normal configuration as no force is applied thereto. When thisinterferometric modulator 501 is driven, the deformable layer 506deforms into the driven state illustrated in FIG. 23B. As the deformablelayer 506 is deforming to its deformed state, the deformable layer 506first contacts the tip 510 of the clips 509 and compresses the tip 510into the substantially flat configuration as shown in FIG. 23B. Thespring clips 509 in their flat configuration have a tendency to returnto their normal configuration. This tendency produces a force that isexerted by the tips 510 on the deformable layer 506. When actuating thedeformed layer 506 from the deformed state to its flat state, the forceof the spring clips 509 exerted on the deformable layer 506 can help theactuation and increase the likelihood and/or speed of the recovery ofthe deformable layer 506.

The embodiment illustrated in FIGS. 23C and 23D is the same as theembodiment of FIGS. 23A and 23B except that the spring clips 509 areformed on the deformable layer 506. In the embodiments of FIGS. 23A-23D,the spring clips 509 can also serve as the above-described landing padsand/or bumps that maintain a desired distance between the stationarylayer 502 and the deformable layer 506.

FIGS. 23E and 23F illustrate another embodiment of the interferometricmodulator 501 that includes the spring clips 509. Referring to FIG. 23Ewhich illustrates the interferometric modulator 501 in the undrivenstate, the stationary layer 502 has a recess 520 and the spring clip 509has a portion contained in and attached to the recess 520. The tip 510of the spring clip 509 is bent with respect to the portion of the clip509 contained in the recess 520 and extends upwardly beyond the topsurface of the stationary layer 502 into the interferometric cavity 418.Referring to FIG. 23F illustrating the driven state, the tip 510 of thespring clip 509 is substantially flattened by the deformable layer 506and the stationary layer 502. Again, this tip 510 has the tendency toreturn to its normal configuration shown in FIG. 23E and thus exerts aforce on the deformable layer 506 that is in the direction away from thestationary layer 502.

In the embodiment of FIGS. 23E and 23F, the thickness 521 of the springclip 509 is substantially the same as or smaller than the depth of therecess 520. As a result, the deformable layer 506 contacts the topsurface of the stationary layer 502 in the driven state as shown in FIG.23F. In another embodiment, the thickness 521 of the spring clip 509 atthe tip 510 and/or in the portion contained in the recess 520 may begreater than the depth of the recess 520. In such an embodiment, in thedriven state of the interferometric modulator 501, the deformable layer506 contacts the spring clip 509 particularly at the area thereof thathas the thickness 521 greater than the depth of the recess 520, whilenot contacting the stationary layer 502. In this configuration, thespring clips 509 serve as the above-described landing pads and/or bumpsas well as the spring clips 509 prevent direct contact between thestationary layer 502 and the deformable layer 506.

As will be appreciated by one of skill in the art, the spring clips 509may not have the exact configuration as illustrated in FIGS. 23A-23F.Also, many different types of biasing mechanisms and springs may beemployed in lieu of the clips 509. Additionally, materials with biasingcharacteristics can also be employed. For example a landing pad thatincludes one or more elastomeric materials may also be employed in lieuof the clips 509. For the sake of convenience, the term “spring clip”refers to any and all mechanisms having the function of exerting a forceon the deformable layer 506 in the direction toward its undriven state.Although two spring clips 509 are illustrated in FIGS. 23A-23F, a singlespring clip or more than two spring clips may be employed. Optionally,two or more spring clips 509 are arranged in the interferometric cavity418 such that the forces exerted on the deformable layer 506 by thespring clips 509 are substantially balanced with one another, ratherthan focusing the forces on a local area of the deformable layer 506.

As will be appreciated by one of skill in the art, the size, placementand strength of the spring or biasing elements can all be variedaccording to the desired characteristics of the interferometricmodulator. The stronger the spring, the faster and the more reliably thedeformable layer 506 will return to its undriven planar position. Ofcourse, this may also require one to adjust the initial voltage input inorder to drive the interferometric modulator 501 to its fully drivenstate, as the deformable layer 506 will tend to have an increased amountof resistance against the spring clips 509 during its approach towardsthe stationary layer 502.

In some embodiments, the spring clips 509 are useful in overcomingstictional forces (static friction) that may develop when the deformablelayer 506 comes in close proximity to or contacts the stationary layer502. These forces can include Van der Waals or electrostatic forces, aswell as other possibilities as appreciated by one of skill in the art.The stictional forces in nature hinder the separation of the deformablelayer 506 from the stationary layer 502. Since the spring clips 509provide additional force to separate the deformable layer 506 from thestationary layer 502, the force of the spring clips 509 can balance orovercome the stictional forces.

In some embodiments, the stictional forces between the deformable layer506 and the stationary layer 502 can be reduced by coating the layerswith a polymer that reduces static friction with or without the springclips. For example, the layers can be coated by an anti-stiction polymercoating, which can reduce the degree of adhesion between the deformablelayer 506 and the stationary layer 502. In one embodiment, this coatingis applied to other aspects of the device, such as the spring clips 509,bumps 511 or landing pad 513.

As will be appreciated by one of skill in the art, the above features oflanding pads 513, bumps 511 and spring clips 509 may be employedindividually or may be employed together in a single embodiment. Forexample, an interferometric modulator may have one, two or all three ofthese features. Also, as described, certain features can serve both toassist in the return of the deformable layer 506 to its undriven stateand to reduce the likelihood that the deformable layer 506 and thestationary layer 502 adversely contact each other, as landing pads 513and spring clips 509 might function.

Multi-State Interferometric Modulators

In some embodiments, the interferometric modulator provides more thantwo states (driven and undriven). An example of this is illustrated inthe embodiment shown in FIGS. 24A-24C. In this embodiment, theinterferometric modulator is not only capable of a deflection of thedeformable layer 506 towards the layer 503, in the driven state as shownin FIG. 24B, but the interferometric modulator is also capable ofreversing the direction of the deflection of layer 506 in the oppositedirection, as illustrated in FIG. 24C. This “upwardly” deflected statemay be called the “reverse driven state.”

As will be appreciated by one of skill in the art, this reverse drivenstate can be achieved in a number of ways. In one embodiment, thereverse driven state is achieved through the use of an additionalstationary layer 502′ that can pull the deformable layer 506 in theupward direction, as depicted in FIG. 24C. In this particularembodiment, there are basically two interferometric modulatorspositioned symmetrically around a single layer 506. This allows each ofthe stationary layers 502 and 502′ to attract the layer 506 in oppositedirections. Thus, while an initial voltage command may send layer 506into the normal driven state (FIG. 24B), the next voltage command canaccelerate the recovery of the deformable layer 506 by driving thatlayer towards the reverse driven state. In this mode, the deformablelayer 506 is then attracted in the opposite direction to the stationarylayer 502′. In this embodiment, the stationary layers 502 and 502′ maybe in various constructions as described earlier in the disclosure, anddo not have to be in the same construction at the same time. Forexample, the stationary layers 502 and 502′ can be in a single layerconstruction or in multiple sub-layer construction. In the illustratedembodiment, a support surface 500′ is maintained some distance above thedeformable layer 506 through a second set of supports 504′.

As will be appreciated by one of skill in the art, not all of theseelements will be required in every embodiment. For example, if theprecise relative amount of upward deflection, such as that shown in FIG.24C compared to FIG. 24A or 24B, is not relevant in the operation of thedevice, then the stationary layer 502′ can be positioned at variousdistances from the deformable layer 506. Thus, there may be no need forsupport elements 504′ or a separate substrate 500′. In theseembodiments, it is not necessarily important how far upward thedeflection of the deformable layer 506 extends, but rather that thestationary layer 502′ is configured to attract the deformable layer 506at the appropriate time. In other embodiments, the position of thedeformable layer 506 as shown in FIG. 24C may alter opticalcharacteristics of the interferometric modulator. In these embodiments,the precise distance of deflection of layer 506 in the upward directioncan be relevant in improving the image quality of the device.

As will be appreciated by one of skill in the art, the materials used toproduce the stationary layer 502′ (or its sub-layers) and substrate 500′need not be similar to the materials used to produce the correspondinglayer 502 and substrate 500. For example, in some embodiments, lightneed not pass through the layer 500′ while it may be necessary for lightto be able to pass through the layer 500. Additionally, if layer 502′ ispositioned beyond the reach of layer 506 in its deformed upwardposition, then a dielectric sub-layer may not be needed in thestationary layer 502′ as there is little risk of layer 506 contactingthe conductive portion of the layer 502′. Accordingly, the voltagesapplied to layers 502′ and 506 can be different based on the abovedifferences.

As will be appreciated by one of skill in the art, the voltage appliedto drive the deformable layer 506 from the driven state shown in FIG.24B to the undriven state shown in FIG. 24A, may be different from thatrequired to drive the deformable layer 506 from the state shown in FIG.24A to the upward or reverse driven state shown in FIG. 24C, as thedistance between plates 502′ and 506 is different in the two states.Thus, the amount of voltage to be applied is determined based upon thedesired application and amounts of deflection.

In some embodiments, the amount of force or the duration that a force isapplied between the layer 502′ and the layer 506 is limited to that isnecessary to merely increase the rate at which the interferometricmodulator transitions between the driven state and the undriven state.Since the deformable layer 506 can be made to be attracted to either thelayer 502 or 502′ which are located on opposite sides of the layer 506,a very brief driving force can be provided to weaken the interaction ofthe layer 506 with the opposite layer. For example, as the layer 506 isdriven to interact with the layer 502, a pulse of energy to the oppositelayer 502′ can be used to weaken the interaction of the layer 506 withthe layer 502′ and thereby make it easier for the deformable layer 506to move to the undriven state.

Controlling Offset Voltages

Traditionally, interferometric modulator devices have been designed suchthat there is a minimum, or no, fixed electrical charge associated witheach layer. However, as current fabrication techniques have not beenable to achieve a “no fixed charge standard,” it is frequently desirableto have the resulting fixed charge considered and compensated for whenselecting the operational voltages used to control the deformable layer506.

Through testing various configurations of layers and various depositiontechniques, the amount of fixed electrical charge that is associatedwith each layer can be modeled and used as design criteria to selectmaterials and layer configurations that minimize the amount of totaloffset voltage imparted to the interferometric modulator. For example,one or more materials can be replaced in the interferometric modulatorlayers to change the electrical characteristics of the overallinterferometric modulator device.

Referring now to FIG. 24D, in some embodiments, the dielectric sub-layer413 or another sub-layer of the stationary layer 502 is modified with acharged component in order to obtain a neutrally charged system. In theillustrated embodiment, the stationary layer 502 is in a two sub-layerconstruction, a dielectric sub-layer 413 is located on a sub-layer 416that serves as mirror and conductive electrode, and the dielectricsub-layer 413 contains charged components 514. Again, the stationarylayer 502 can be in various constructions as described above.

The incorporation of the charged component 514 can be achieved in anumber of ways. For example, additional charged components 514 can beadded to the dielectric material when the dielectric sub-layer 413 isbeing formed on the underlying sub-layer 416. As will be appreciated byone of skill in the art, there are a variety of charged components thatcan be used, the amount and particular characteristics of these chargedcomponents can vary depending upon the desired properties of theinterferometric modulator. Examples can include, forming a dielectriclayer in a sputter tool (which can be negative) as compared to achemical vapor deposition process (which can be positive), or alteringthe amount of hydrogen in the layer.

In some embodiments, the control of the amount of charged components 514in the interferometric modulator can also be achieved through alteringthe method of deposition of the layers or adding entirely new layers. Inanother embodiment, one selects particular materials with the goal ofoptimizing the electrochemical characteristics of the materials. Thus,one can use various work function differences to control the finaloffset voltage of the interferometric modulator or change the chargeaccumulation rate within the device during operation of the device. Forexample, the deformable layer 506 can have a surface that can contactthe stationary layer 502, the surface can have a high work function tominimize the transfer of electrons between the layers. In anotherembodiment, one can modify a sacrificial material used in the creationof the interferometric modulator so that as the sacrificial material isbeing removed, one is not imparting charge to the deformable layer 506and/or the stationary layer 502. In another embodiment, materials to beused to connect the layers 502 and 506 during processing can be selectedon the basis of their work function properties. In another embodiment,the material selected for the connector rod 333 (FIGS. 25A and 25B) isbased on its work function characteristics.

In one embodiment, during the creation of the interferometric modulator,the stationary layer 502 and the deformable layer 506 are electricallyconnected so as to minimize the charge difference between the twolayers. This can allow for higher yield in production and higherreliability in the final interferometric modulator. This electricalconnection can be removed to allow the device to properly function. Inone embodiment, this connection between the two layers is created fromthe same material as that from which the deformable layer 506 iscreated.

Reducing the Movement of the Deformable Layer 506

In some embodiments, the supports 504 interact with the deformable layer506 through direct contact of the top end 37 of the supports 504 and thebottom surface of layer 506. In certain situations, sliding or slippageof the deformable layer 506 along the top 37 of support 504 may occur.This movement can be decreased in a number of ways. In one embodiment,the movement is decreased by altering the surface characteristics of thetop 37 of the support 504. For example, one can roughen the deformablelayer 506 and/or the support 504 at the point 505 where the twointeract, as shown in FIGS. 24D and 249E. For example, this can be doneby oxygen plasma burn down of the support or by sputter etching beforethe deposition of the deformable layer 506.

Alternative Forces for Driving Recovery from the Driven State

In some embodiments, the manner of deformation of the deformable layer506 may be altered for improved functionality. In a traditionalinterferometric modulator 501, the deformable layer 506 is a singlecontiguous sheet stretched taut across the support members 504. Becausethe layer is stretched taut, the residual stress in the layer allows thelayer to “spring” or “snap back” from the driven state to the undrivenstate. However, this particular arrangement can be sensitive to processvariability.

Instead of relying upon the tautness of the deformable layer 506 (tocreate residual stress), one can instead rely upon the elastic modulusof the material, which is a constant based upon the material, ratherthan on primarily how the material is arranged or processed. Thus, inone aspect, the deformable layer 506 retains and provides its elasticitythrough a material constant of the material from which it is made. Inone embodiment, this is similar to that of a cantilever spring, ratherthan a taut stretched film. An example of such a design is shown inFIGS. 25A-25D. FIG. 25A shows a side view, and FIG. 25B shows a top viewof one embodiment of an interferometric modulator 501 in the undrivenstate. FIG. 25C shows a side view and FIG. 25D shows a top view of theinterferometric modulator 501 in a driven state.

In this embodiment, the deformable layer 506 has been divided into twoseparate parts, a load bearing part 506 a that is responsible forproviding the flexibility and resilience for the movement of the layerthrough its elastic modulus, and a substantially planar part 506 b,which functions as the secondary mirror for the interferometricmodulator. The two parts 506 a and 506 b are connected to each other viaa connector rod 333. In one embodiment, the connector rod 333 is made ofthe same material as the load bearing part 506 a and/or thesubstantially planar part 506 b. In another embodiment, the connectorrod 333 is made of a material different from the load bearing part 506 aand the substantially planar part 506 b. In some embodiments, theconnector rod 333, rather than the load bearing structure 506 a, is thepart that provides flexibility and resilience to the system. In someembodiments, the load bearing structure 506 a is thicker than thedeformable layer 506 in the previous embodiments.

As shown in FIG. 25B, the load bearing part 506 a is configured in an“X” shape that is supported at its four corners 70, 71, 72, and 73 toprovide its elastomeric properties. In the driven state, the loadbearing part 506 a bends downward and towards the stationary layer 502through the pull from the planar part 506 b of the deformable layer 506.As will be appreciated by one of skill in the art, the particularmaterial or materials used to provide the elasticity for the system canvary depending upon the particularly desired characteristics of thesystem.

The above-described modifications can help remove process variabilityand lead to a more robust design and fabrication. Additionally, whilethe above aspects have been described in terms of selected embodimentsof the interferometric modulator, one of skill in the art willappreciate that many different embodiments of interferometric modulatorsmay benefit from the above aspects. Of course, as will be appreciated byone of skill in the art, additional alternative embodiments of theinterferometric modulator can also be employed. The various layers ofinterferometric modulators can be made from a wide variety of conductiveand non-conductive materials that are generally well known in the art ofsemi-conductor and electromechanical device fabrication.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. As will be recognized, the present invention may be embodiedwithin a form that does not provide all of the features and benefits setforth herein, as some features may be used or practiced separately fromothers.

1. A method of making a microelectromechanical system (MEMS) device,comprising: forming a first electrode; depositing a dielectric materialover at least a portion of the first electrode; removing a portion ofthe dielectric material from over the first electrode, thereby forming avariable thickness dielectric layer; and forming a second electrode overat least a portion of the variable thickness dielectric layer.
 2. Themethod of claim 1, further comprising depositing a sacrificial layerover at least a portion of the dielectric material.
 3. The method ofclaim 2 in which the MEMS device comprises an interferometric modulator.4. The method of claim 2 in which the second electrode comprises asurface facing the variable thickness dielectric layer, the surface ofthe second electrode facing the variable thickness dielectric layerhaving an average peak-to-valley surface profile variation that is lessthan an average peak-to-valley surface profile variation of the variablethickness dielectric layer.
 5. The method of claim 1 in which thedielectric material comprises at least a first layer and a second layer.6. The method of claim 5 in which the first layer has a first thicknessthat is greater than a second thickness of the second layer.
 7. Themethod of claim 6 in which the first thickness is in the range of about200 Å to about 3000 Å.
 8. The method of claim 6 in which the secondthickness is in the range of about 50 Å to about 500 Å.
 9. The method ofclaim 5, further comprising patterning the dielectric material to definestops.
 10. The method of claim 9 in which removing a portion of thedielectric material comprises removing a portion of the second layer ofthe dielectric material such that the stops remain.
 11. The method ofclaim 10, further comprising depositing a sacrificial layer over atleast a portion of the dielectric material.
 12. The method of claim 11,further comprising removing the sacrificial layer and at least a portionof the first layer of the dielectric material.
 13. The method of claim12 in which removing the sacrificial layer and the at least a portion ofthe first layer of the dielectric material further comprises etchingwith an etchant.
 14. The method of claim 13 in which etching furthercomprises removing the first layer of the dielectric material at a firstetch rate that is higher than a second etch rate for removing the secondlayer.
 15. The method of claim 1 in which the dielectric material iscompositionally graded.
 16. The method of claim 15 in which thedielectric material is a graded dielectric material selected from thegroup consisting of graded silicon oxide and graded silicon nitride. 17.The method of claim 16 in which the graded dielectric material at aninterface with the first electrode is enriched in Si relative to theoverall composition of the graded dielectric material.
 18. The method ofclaim 1, further comprising depositing an intermediate layer over the atleast a portion of the first electrode.
 19. The method of claim 18 inwhich the intermediate layer comprises at least one of an optical layer,a barrier layer or a non-conductive layer.
 20. The method of claim 18,comprising depositing the dielectric material over the intermediatelayer.
 21. A method of making an interferometric modulator, comprising:forming a first electrode; depositing a dielectric layer over at least aportion of the first electrode; removing a portion of the dielectriclayer to form a variable thickness dielectric layer; depositing asacrificial layer over the variable thickness dielectric layer;planarizing the sacrificial layer; and forming a second electrode overthe sacrificial layer.
 22. The method of claim 21 further comprisingforming a planarization layer over the sacrificial layer.
 23. The methodof claim 21 further comprising removing the sacrificial layer.
 24. Themethod of claim 21 in which the variable thickness dielectric layercomprises at least one stop.
 25. The method of claim 21 in which thesecond electrode comprises a lower surface at an interface with thesacrificial layer, the lower surface of the second electrode having anaverage peak-to-valley surface profile variation that is less than anaverage peak-to-valley surface profile variation of the variablethickness dielectric layer.
 26. A method of making an interferometricmodulator, comprising: forming a first electrode; depositing adielectric layer over at least a portion of the first electrode;removing a portion of the dielectric layer to form a variable thicknessdielectric layer; depositing a sacrificial layer over the a variablethickness dielectric layer; depositing a planarization layer over thesacrificial layer; and forming a second electrode over the planarizationlayer.
 27. The method of claim 26 further comprising removing thesacrificial layer.
 28. The method of claim 26 further comprisingplanarizing the sacrificial layer.
 29. The method of claim 26 in whichthe variable thickness dielectric layer comprises at least one stop. 30.The method of claim 26 in which the second electrode comprises a lowersurface at an interface with the planarization layer, the lower surfaceof the second electrode having an average peak-to-valley surface profilevariation that is less than an average peak-to-valley surface profilevariation of the variable thickness dielectric layer.